Does рассвет = рас + свет, "the spreading or diffusion of light (at the beginning of the day)"?
Even if it doesn't mean that exactly, it's still a useful mnemonic for remembering it. :-)
Is there a different way to say "it will get cold at dawn" since it does not accept that as an answer here?
На рассвете похолодает, I guess.
Or "на рассвете станет холодно", but Djenthallman's version sounds better.
can it be both dawn and twilight?
No, "twilight" is "сумерки". "Рассвет" doesn't have this meanìng.
thanks for putting it clear. at first the two options were given.